The present invention relates to data packet forwarding networks, being more particularly concerned with labeled packets normally routed over predetermined primary label switched paths in which a packet passes from one switch or switching entity to another, and wherein the label of an incoming packet identifies the next switch or switching entity to which the packet is to be forwarded along such primary path.
In a number of current data packet forwarding networks, the packets are routed over predetermined pre-computed unidirectional packet switching paths according to labels attached to the packets, such that as a packet passes from one switch or switching entity in the network path to another switch or switching entity, the label of an incoming packet, as before stated, identifies the next switch or switching entity in the path to which the packet is to be forwarded. Among such packet forwarding system are Asynchronous Transfer Mode systems (ATM), Frame Relay (FR) and Multi-Protocol Label Switching (MPLS) networks.
In the forwarding operation of such systems, the incoming data packet label is replaced with the label known to the next hop switch or switching entity along the predetermined routing path before the packet is forwarded there along. Such path is often referred to as the label switched path, and it consists of one or more virtual links each linking adjacent switches along the path. The labeled switch path is typically established by means of a signaling protocol that binds each virtual path link with a particular label value that uniquely identifies a given link to switches at each end of the path link. In addition, it may be noted that a switching entity can be a single packet switching device or, at a higher abstraction level, a group of switching devices that can be viewed as a single entity.
When an established label switched path becomes unusable (e.g. due to a link failure or congestion), data has to be re-routed over an alternative path. Such an alternative path can be established after a primary path failure is detected, or, alternatively, it can be established beforehand in order to reduce the path switchover time. Pre-established alternative paths are essential, indeed, where packet loss due to a link failure is undesirable.
Since, however, it may take a significant time delay for a device on a label switched path to detect a distant link failure, it may continue sending packets along the primary path. As soon as such packets reach a switch that is aware of the failure, packets must be immediately rerouted by switching to an alternative path away from the failed link; if loss of data is to be avoided. Since it is impossible to predict where failure may occur, multiple alternative label switched paths originating at every switch along the primary label switched path might be established to avoid such packet loss.
It is to this problem of setting an alternative label switched path to allow for a fast or rapid reroute of packet flow traffic around a failure or traffic congestion in a primary label switched path, and with minimal signaling requirements and alternative path computation complexity, and with efficient utilization of network resources, that the present invention is directed.
It is accordingly a primary object of the invention to provide a new and improved method of and apparatus for fast alternate-path rerouting of labeled data packets normally, routed over a predetermined primary label switched path, upon failure or congestion in the primary path, and that shall not be subject to the before-mentioned prior art link failure-detection time delays or the use of multiple alternative label switch paths at every switch, or alternate-path computation and/or signaling requirement complexities; but that, to the contrary, ensures a fast reroute with highly efficient utilization of network resources.
A further object is to provide for in-band quick detection of link failures or congestion along a primary path without resorting to an out-of-band signaling mechanism.
Other and further objects will be explained hereinafter and are more particularly delineated in the appended claims.
In summary, however, from one of its important aspects, the invention embraces in a label-switched data packet forwarding network comprising a predetermined primary routing path for unidirectional packet flow traffic along successively linked switching entities defining such primary path in the direction between a source ingress end switch and a destination egress end switch, a method of protecting said primary path from a failure or congestion in the path, that comprises, setting up an alternative label switched path segment between said source ingress and destination egress end switches of the primary path, separate from the primary path and sharing no link or switching entity of the primary path apart from the ingress and egress end switches, and between which the alternative label switched path segment is externally connected, and, upon the occurrence of a failure or congestion between switching entities of the protected primary path, reversing the packet flow traffic in the primary path back in the opposite direction towards the source ingress switch as an initial reverse alternate path segment, and thence along said external alternate label switched path segment to the egress switch, thereby rerouting the packet flow traffic around the primary path through said external alternative label switched path segment.
Preferred and best mode design and embodiments of the invention are later described in detail.